'h' ion in OLM cell

in `set_chanparams()` there was no 'h' ion (used by HCN channels) inserted.
- from OLM cell template https://github.com/mbezaire/ca1/blob/development/cells/class_olmcell.hoc#L195
I've solved by `insert h_ion` and set reversal with `eh = -32.9`